"I let them know I was extremely unhappy with what was happening and they have substantially upped their commitment and now we're very happy and have a very, very powerful, very very strong NATO, much stronger than it was two days ago," he said in a press conference.

He again called himself a "very stable genius" and took "total credit" for what he said was a $33 billion (A$45 billion) increase in NATO defence spending this year.

But French president Emmanuel Macron denied any change in policy as a result of Mr Trump's involvement, saying the allies had simply reaffirmed their earlier commitment to increase defence spending to two per cent of gross domestic product by 2024.

Other leaders said they had simply told Mr Trump they were open to discussing spending increases in the future.

Presidents have been trying unsuccessfully for years to get Germany and other rich NATO Nations to pay more toward their protection from Russia. They pay only a fraction of their cost. The U.S. pays tens of Billions of Dollars too much to subsidize Europe, and loses Big on Trade!

Mr Trump is annoyed because only the US, Britain, Estonia and Greece currently meet the two per cent target, with America spending the most at 3.5 per cent.

During a tense summit, the businessman announced his wish that all 29 member nations should increase spending to two per cent immediately, and eventually double that to four per cent of GDP.

NATO Secretary General Jens Stoltenberg was forced to call an emergency budget meeting of all the allies to deal with the staggering demand.

Just eight countries are forecast to reach the two per cent goal this year, and 15 by 2024. But Mr Trump seemed satisfied they had agreed to meet the target soon.

"Ultimately, that will be going up quite a bit higher than that," he said.

Billions of additional dollars are being spent by NATO countries since my visit last year, at my request, but it isn’t nearly enough. U.S. spends too much. Europe’s borders are BAD! Pipeline dollars to Russia are not acceptable!

A source briefed on Mr Trump's meeting with fellow NATO nations told the New York Times the President had said that if the other countries did not meet the target by January, the US "would go it alone."

But Mr Macron said his US counterpart had not threatened to leave, insisting: "President Trump never at any moment, either in public or in private, threatened to withdraw."

Mr Trump said his commitment to NATO was "very strong", signing a declaration reaffirming the group's commitments - despite the contrast with many of his stated views.

Mr Trump pumps his fist during his arrival in Regent's Park, London, to spend the night at the residence of the US Ambassador. Picture: AP Photo/Pablo Martinez Monsivais

UK TENSIONS AND PUTIN 'COMPETITION'

The President singled out Germany for particular criticism over low spending, accusing the nation of being a "captive" of Russia because of a multibillion-dollar pipeline deal.

At the same time, he insisted Vladimir Putin was a "competitor" rather than an "enemy", ahead of their meeting in Finland next week after his four-day trip to the UK.

What good is NATO if Germany is paying Russia billions of dollars for gas and energy? Why are there only 5 out of 29 countries that have met their commitment? The U.S. is paying for Europe’s protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025.

He vowed to bring up Syria and allegations of Russian meddling in the US presidential elections, which his campaign is accused of colluding in. But he did not rule out recognising Rissia's annexation of Crimea.

The President said his meeting with the Russian leader may be the easiest" part of his European tour, with massive protests expected over his presence in London.

The British government is also gripped by a crisis over Brexit and tension with Russia after the UK blamed Moscow for this month's death of a woman from a Novichok nerve agent.

Mr Trump fuelled a fire by appearing to criticise prime minister Theresa May's Brexit plan, after the resignation of foreign secretary Boris Johnson and her minister in charge of the EU exit, David Davis. "The people voted to break it up (Britain's ties with the EU)," said Mr Trump.

"So I would imagine that's what they will do, but maybe they will take a little bit of a different route. I don't know that is what they voted for."